cmd/compile: enable inlining variadic functions

As a side effect of working on mid-stack inlining, we've fixed support
for inlining variadic functions. Might as well enable it.

Change-Id: I7f555f8b941969791db7eb598c0b49f6dc0820aa
Reviewed-on: https://go-review.googlesource.com/100456Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/inl.go

@@ -13,7 +13,7 @@
 //      0: disabled
 //      1: 80-nodes leaf functions, oneliners, lazy typechecking (default)
 //      2: (unassigned)
-//      3: allow variadic functions
+//      3: (unassigned)
 //      4: allow non-leaf functions
 //
 // At some point this may get another default and become switch-offable with -N.
@@ -23,9 +23,6 @@
 //
 // The debug['m'] flag enables diagnostic output.  a single -m is useful for verifying
 // which calls get inlined or not, more is for debugging, and may go away at any point.
-//
-// TODO:
-//   - inline functions with ... args
 
 package gc
 
@@ -141,17 +138,6 @@ func caninl(fn *Node) {
 		Fatalf("caninl on non-typechecked function %v", fn)
 	}
 
-	// can't handle ... args yet
-	if Debug['l'] < 3 {
-		f := fn.Type.Params().Fields()
-		if len := f.Len(); len > 0 {
-			if t := f.Index(len - 1); t.Isddd() {
-				reason = "has ... args"
-				return
-			}
-		}
-	}
-
 	// Runtime package must not be instrumented.
 	// Instrument skips runtime package. However, some runtime code can be
 	// inlined into other packages and instrumented there. To avoid this,

test/inline_variadic.go

-// errorcheck -0 -m -l=3
+// errorcheck -0 -m
 
 // Copyright 2016 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Test more aggressive inlining (-l=3 allows variadic functions)
+// Test inlining of variadic functions.
 // See issue #18116.
 
 package foo